1. Field of the Invention
The present invention generally relates to an optical scanning device used in an image forming device, and specifically relates to optical scanning devices that respectively scans different scanning-subject surfaces using a respective optical beam.
2. Description of the Related Art
An optical scanning device, widely known in relation to laser printers and the like, generally has a following configuration. The optical scanning device deflects an optical beam emitted from a light source using a light deflector. The optical scanning device concentrates light onto a scanning-subject surface using an optical scanning and imaging system, such as an fθ (f-theta) lens, and forms a light spot on the scanning-subject surface. The optical scanning device performs an optical scan (main scan) on the scanning-subject surface using the light spot. The scanning-subject surface is a photosensitive surface of a photosensitive medium that is, for example, a photoconductive photoreceptor in an electronic image forming device.
An example of a full-color image forming device is a following electronic image forming device. In the electronic image forming device, four photoreceptors are arrayed in a direction in which a recording paper is carried. A single deflecting unit deflection-scans light beams emitted from a plurality of light source units. The light source units are provided in correspondence with each photoreceptor. A plurality of optical scanning and imaging systems corresponding to each photoreceptor simultaneously perform exposure on each photoreceptor and form latent images. A developing unit visualizes each latent image using developers of differing colors, such as yellow, magenta, cyan, and black. The visualized images are successively superimposed and transferred onto a same sheet of recording paper. Then, the electronic image forming device fixes the transferred images and obtains a color image.
The image forming device using two sets or more of a combination of the optical scanning device and the photoreceptor in this way to obtain a two-color image, a multi-color image, a color image, or the like is known as a tandem-type image forming device. Such tandem-type image forming devices can be configured so that the, photoreceptors share a single light deflector.
FIG. 14A is a perspective view of a configuration of a conventional tandem-type image forming device. FIG. 14B is an enlarged perspective view of a main section of a light source periphery shown in FIG. 14A.
In FIG. 14A, light beams L are emitted from four light sources 101a to 110d and incident on a polygon mirror 102. The polygon mirror 102 is a single light deflector. Each light beam L is concentrated onto four photoreceptor drums 104 as light spots, via an optical system 103. The light spots scan photosensitive surfaces of the photoreceptor drums 104. The photoreceptor drums 104 are scanning-subject surfaces corresponding to each following color: cyan (C), magenta (M), yellow (Y), and black (K). The optical system 103 includes a coupling lens, a cylinder lens, a scanning lens, a reflecting mirror, and the like.
FIG. 14B is an enlarged view of a periphery of the light sources 101a to 101d and the polygon mirror 102 in FIG. 14A. The optical scanning device has an opposed-scanning method configuration in which the light beams L are irradiated onto deflective reflecting surfaces facing the polygon mirror 102.
In Japanese Patent No. 3295281, a following configuration is described. A plurality of light beams that are roughly parallel and separated in a sub-scanning direction are incident on a light deflector. A plurality of scanning optics corresponding to the light beams are arrayed in the sub-scanning direction and perform a scan. In Japanese Patent Application Laid-open No. 2001-4948, Japanese Patent Application Laid-open No. 2001-10107, and Japanese Patent Application Laid-open No. 2001-33720, a following configuration is described. Light beams are incident from one side of a light deflector. An optical scanning system has a three-lens structure. A plurality of light beams moving toward different scanning-subject surfaces pass through scanning lenses L1 and L2. A scanning lens L3 is provided for each scanning-subject surface.
In configurations such as these, in which the light deflector is shared among a plurality of scanning-subject surfaces, the image forming device can be made compact by a reduction in a number of light deflectors.
The number of light deflectors can be reduced in the optical scanning device of a full-color image forming device having, for example, four different scanning-subject surfaces (photoreceptors) corresponding to cyan, magenta, yellow, and black. However, because the light beams moving toward the photoreceptors in the sub-scanning direction are lined in a rough parallel and incident on the light deflector, the polygon mirror forming the light deflector is enlarged in the sub-scanning direction. Generally, among optical elements included in the optical scanning device, the polygon mirror is expensive. The expensiveness of the polygon mirror interferes with a reduction in cost and a reduction in size of the overall device.
In the optical scanning device of the color image forming device, an oblique incidence optical system has recently become known as a means for achieving reduced cost through use of the single light deflector. In the oblique incidence optical system, the light beams are incident on the deflective reflecting surface of the light deflector in the sub-scanning direction at an angle (refer to Japanese Patent Application Laid-open No. 2003-5114).
In the oblique incidence optical system, the light beams are respectively deflected and reflected by the deflective reflecting surface of the light deflector. Then, the light beams are separated by a reflecting mirror or the like and respectively led to corresponding scanning-subject surfaces. In this configuration, an angle of each light beam in the sub-scanning direction (angle at which the light beam is obliquely incident on the light deflector) is set to an angle allowing the reflecting mirror to separate each light beam.
Through the use of the oblique incidence optical system, space between adjacent light beams in the sub-scanning direction allowing the reflecting mirror, to separate each light beam can be secured without an increase in light reflector size (increase in stages and thickness of the polygon mirror in the sub-scanning direction).
However, in an oblique incidence method used in the above-described oblique incidence optical system, image height variations in beam spot diameters increase because of an occurrence of scanning line curvature and deterioration of wavefront aberration.
An amount of scanning line curvature occurrence differs depending on an oblique incidence angle of each light beam in the sub-scanning direction. When latent images written using each light beam are superimposed and visualized using toners of each color, the scanning line curvature appears as color shifts. The light beam is incident on the scanning lens in a twisted state because the light beam is obliquely incident. As a result, the wavefront aberration increases, optical performance, particularly in peripheral image heights, is significantly deteriorated, and the beam spot diameter is widened, thereby interfering with formation of higher-quality images.
In an oblique incidence method, the light beams from the light sources are incident toward a rotation axis of the polygon mirror. Therefore, when the light sources are disposed in positions overlapping with an optical axis of the scanning lens in a main scanning direction, the oblique incidence angle is increased to avoid interference with the scanning lens.
As a method for correcting significant scanning line curvature that is a phenomenon unique to the oblique incidence method, following configurations are proposed. For example, an optical scanning and imaging lens system includes a lens having a lens surface that shifts an inherent tilt of the lens surface within a sub-scanning cross-section in a main scanning direction to correct the scanning line curvature (refer to Japanese Patent Application Laid-open No. H11-14932). Alternatively, an optical scanning and imaging system includes a corrective reflecting surface having a reflecting surface that shifts an inherent tilt of the reflecting surface within the sub-scanning cross-section in the main scanning direction to correct the scanning line curvature (refer to Japanese Patent Application Laid-open No. H11-38348).
In Japanese Patent Application Laid-open No. 2004-70109, a following configuration is described. An obliquely incident light bundle passes off-axis of the scanning lens. Positions of scanning lines are aligned using a surface that changes an amount of aspheric surfaces of child lines of the scanning lens in the sub-scanning direction. In Japanese Patent Application Laid-open No. 2004-70109, an example is given in which the scanning line curvature correction is performed in one scanning lens. In this example, although the scanning line curvature can be corrected, degradation of the beam spot diameter caused by increased wavefront aberration, described hereafter, is not described.
Another problem regarding the oblique incidence method is that significant deterioration of the wavefront aberration easily occurs in peripheral image height (near both ends of the scanning lines) because of skew ray. When such wavefront aberrations occur, spot diameters of the light spots in the peripheral image height become widened. In other words, variations in beam spot diameters among image heights occur. If this problem cannot be solved, high-quality images that have been in strong demand in recent years cannot be achieved. In the optical scanning devices described in Japanese Patent Application Laid-open No. H11-14932, Japanese. Patent Application Laid-open No. H11-38348, and Japanese Patent Application Laid-open No. 2004-70109, significant scanning line curvature unique to the oblique incidence method is successfully corrected. However, the correction of the wavefront aberration is insufficient.
As an optical scanning device that can successfully correct the scanning line curvature and the deterioration of the wavefront aberration that are problematic in the oblique incidence method, as described above, a following optical scanning device is proposed (refer to Japanese Patent No. 3450653). An optical scanning and imaging system includes a plurality of rotating asymmetric lenses. A shape of a mother line connecting vertices of child lines of lens surfaces of the rotating asymmetric lenses is curved in a sub-scanning direction.
However, the lenses having the lens surfaces of which the shape of the mother line connecting the vertices of the child lines is curved in the sub-scanning direction solves the above-described problems by the mother line being curved. As a result, individual scanning lenses corresponding to the incident light beam are required. Therefore, when the lenses are applied to a tandem-type optical scanning system, a number of scanning lenses is increased.
When light beams moving toward different scanning-subject surfaces are incident on a same lens, the above-described problems are solved for one light beam by the shape of the mother line being curved. However, reduction of the scanning line curvature and the wavefront aberration in another light beam becomes difficult.
The curvature of the mother line is in the sub-scanning direction. Therefore, when the light beams incident on the same lens shift in the sub-scanning direction because of influence from an assembly error, a processing error, environmental changes, or the like, a shape of the scanning line curvature changes as a result of influence from a refracting power of the lens in the sub-scanning direction. An initial (or design-stage) color shift control effect on a color image cannot be acquired. As a result, color shifting occurs.
Furthermore, even in the wavefront aberration correction, the changes in a skew state of the light beam are significant on the surface having the curvature because of the incident light beam being blurred. Therefore, stably obtaining a favorable beam spot diameter is difficult.
The scanning line curvature correction is performed in Japanese Patent Application Laid-open No. H11-14932 using the same surface as that in the Japanese Patent No. 3450653. Japanese Patent Application Laid-open No. H11-14932 describes the configuration using the oblique incidence method. However, as described earlier, stably obtaining a favorable beam spot diameter is difficult.
Technological issues of the present invention are to solve the problems of the conventional technology and to:                (1) provide an optical scanning device that can effectively correct the scanning line curvature and the deterioration of the wavefront aberration in an oblique incidence method optical scanning device suitable for achieving low costs, low power consumption, and reduced size; and        (2) provide an image forming device that effectively corrects the scanning line curvature and the deterioration of the wavefront aberration in an oblique incidence method optical scanning device in which color shift occurrence is minimal even during temperature changes.        